Objectives To determine the role of an oviductal glycoprotein, oviductin, or estrogen-dependent glycoprotein, in fertilization. ABSTRACT:Oviductin is a glycoprotein present in oviductal secretions during the periovulatory and early embryonic development periods. In the primate, synthesis and secretion of oviductin is coordinated with the reproductive cycle, being up- regulated by estradiol and down-regulated by progesterone. Oviductins belong to a newly discovered class of mammalian proteins that share homology with chitinases but have no chitinase enzymic activity. For preliminary work (to be confirmed eventually in primates), the function of oviductin has been assayed using a golden hamster fertilization system. This model animal system was chosen because ova are abundant and both ovulated and non-ovulated mature ova can be obtained; early stages of sperm penetration and fertilization are easily assessed; and the large size of the sperm!s head permits regional localization of markers at the light microscopic level. As visualized by indirect immunofluorescence, affinity purified hamster oviductin binds to the zona pellucida of hamster eggs and to the sperm!s head, with regional localization of binding to sperm changing during sperm capacitation. Oviductin binds to sperm both in vitro and in vivo (determined in sperm flushed from oviducts of mated animals). Oviductin profoundly increases the speed of sperm penetration through the zona pellucida the mechanism for this action appears to occur by increased adhesiveness of acrosome reacting sperm for the zona pellucida. Oviductins are heavily glycosylated, with marked variations occurring between species. As determined by probes of transblots with biotinylated lectins, hamster oviductin is strongly positive for N-acetyl-D-glucosamine and alpha-linked-N-acetyl-D-galactosamine, and weakly reactive for alpha-mannose. These preliminary findings in the golden hamster form a strong basis for future highly directed studies of oviductin!s function in primate fertilization and early embryonic development.